It largely depends on what role you want to play. Summoner remains a DPS while Scholar becomes a healer. And in a sense you don't really have to choose because both level up at the same time (being tied to the same beginner class, this is unique to them). The only things you gotta do separately is their individual job quests and their gear (right now there may just be unified caster gear, but eventually you get both caster and healer gear which are very separate, same with their books).